An elevator comprises safety devices, the purpose of which is to prevent dangerous situations related to use of the elevator. This type of safety device is e.g. an electromagnetic brake of an elevator, such as the machinery brake of the hoisting machine of the elevator or a guide rail brake of an elevator car.
Activation of the brake occurs by dropping out the brake by disconnecting the current supply to the magnetizing coil of the electromagnet of the brake. The current traveling in the magnetizing coil decreases with a time constant, the magnitude of which is usually at least some hundreds of milliseconds. The time constant is determined on the basis of, inter alia, the inductance of the magnetizing coil. After the current has decreased sufficiently, the force of the thruster spring in the electromagnetic brake exceeds the force of attraction with which the electromagnet pulls the surfaces of the brake that are on opposite sides of the air gap against each other, after which the brake shoe of a machinery brake or the prong of a guide rail brake starts to move towards the braking surface on a rotating part of the hoisting machine or towards the braking surface on a guide rail of the elevator car. The brake activates when the brake shoe/prong strikes against the braking surface to brake the movement of the hoisting machine/elevator car.
Fast and correctly-timed operation of a brake is important for, inter alia, preventing the drifting of an elevator car away from the stopping floor, because an elevator car drifting away from the stopping floor might cause a shearing hazard for a passenger remaining between the elevator hoistway entrance and the elevator car.
By means of fast and correctly-timed operation of the brake it can also be ensured that the elevator car is not able to collide with the end buffer of the elevator hoistway at an excessive speed. Implementing this is extremely challenging in elevators having a reduced end buffer e.g. owing to shallow top clearances or bottom clearances of the elevator hoistway with respect to the run speed of the elevator car. In this case the brake of the elevator must be activated to brake the speed of an elevator car approaching the end of the elevator hoistway sufficiently early and at precisely the correct moment in order for the speed of the elevator car to have time to decrease to the permitted buffer collision speed before a possible collision with a reduced end buffer. A polyurethane buffer, inter alia, is used as a reduced end buffer, which polyurethane buffer has a rather limited shock absorbing capability and which is also damaged by the force of even a rather small impact.
The operation of the brake can be accelerated by adding a special quenching circuit to the breaker circuit of the current supply, via which quenching circuit the current of the magnetizing coil of the electromagnet of the brake travels during disconnection of the current supply. A quenching circuit comprises one or more components, such as resistors or capacitors, which component(s) receive(s) the energy stored in the inductance of the magnetizing coil while simultaneously accelerating disconnection of the current of the magnetizing coil.
Activation of the brake might be slowed down e.g. owing to failure of the quenching circuit. The activation function of the brake might also be slowed down or even completely prevented as a consequence of, inter alia, a failure of an electronic or electromechanical component of the brake control circuit or of a short-circuit occurring in the brake control circuit, or, for instance, as a consequence of an earth fault of the brake control circuit.